Plasma chambers commonly are used to perform processes for fabricating electronic devices such as semiconductors, displays and solar cells. Such plasma fabrication processes include chemical vapor deposition of semiconductor, conductor or dielectric layers on the surface of a workpiece or etching of selected portions of such layers on the workpiece surface.
It is important for a plasma fabrication process to be performed with high spatial uniformity over the surface of the workpiece. That is, a deposition process should be performed so that the deposited material has uniform thickness and quality at all positions on the surface of the workpiece. Likewise, an etch process should etch material at a uniform rate at all such positions.
One cause of spatial non-uniformity in plasma fabrication processes is the workpiece passageway through which a workpiece can be transferred into or out of the plasma chamber. The workpiece passageway typically is on one side of the chamber wall, which creates an asymmetry in the plasma density above the workpiece. Specifically, in a plasma chamber having an RF powered showerhead, the plasma density is greater adjacent the end of the showerhead closest to the workpiece passageway than it is adjacent the opposite end of the showerhead. The asymmetry in plasma density is undesirable because it produces a corresponding asymmetry in the plasma process (e.g., deposition or etch) being performed on the workpiece.
One design that compensates for the asymmetry in plasma density that otherwise would be caused by the workpiece passageway is disclosed in U.S. patent application Ser. No. 12/343,519 filed Dec. 24, 2008 by Kudela et al., published on Jun. 25, 2009 as US patent application publication 2009-0159423 A1, owned by the same assignee as the present application, the entire disclosure of which is hereby incorporated by reference into the present application. That design supplies RF power to an electrode of the plasma chamber so that the supply of power is offset toward the direction of the workpiece passageway.